Liquid crystal displays (LCDs) with backlight systems occupy an important position among data display devices of various types, both in the volume of production and in respect of their performance characteristics. Such LCDs are widely used in laptop PCs, calculators, mobile telephones, watches, home TV, and many other appliances and devices. The wide range of applications stimulates developers to implement new promising system and technologies. This research and development is aimed, in particular, at increasing the power and efficiency of the backlight systems, reducing their dimensions, and optimizing the spectral and polarization characteristics of radiation backlighting the LCDs.
There are two principal approaches to solving these problems. The first is to increase the intensity of visible light illuminating LCDs, while the second consists in using backlight systems generating partially polarized radiation. The former approach implies either an increase in the power of radiation sources (and, hence, in the weight dimensions and power consumption of the device) or conversion of the ultraviolet (UV) radiation component of the source into visible light. The latter solution implies the use of polarizing prisms, mirrors reflecting light at the Brewster angle, or polarizers of various types.
There are known designs of LCDs in which an increase in the image contrast and brightness is achieved through the use of fluorescent materials and elements based on such materials. For example, U.S. Pat. No. 4,211,473 provides an LCD with enhanced contrast, with pleochroic and fluorescent materials incorporated in one or more components of the device. The materials are used in balanced proportion and possess complementary optical absorption and emission spectra. The radiant flux incident onto the display is converted so that the light reaching the eye from the bright state regions of the display is neutral gray in contrast to the light from the dark state regions, which is strongly attenuated or strongly colored.
Another design disclosed in EP 1,004,921 comprises a liquid crystal layer placed between the front and back substrate plates, with one electrode and one polarizer on each of the plates, and a layer containing a dye. The dye layer is either single-component, emitting in the range of 400-700 nm under the action of UV radiation, or represents a mixture of luminescent and absorbing dyes. The purpose of the invention is to achieve greater brightness, increase color saturation of the image, and expand the viewing angle of LCDs up to 180, by more effectively utilizing the emission spectrum of the radiation source, in particular, in the UV range.
Fluorescent materials are used in LCDs for the correction of color and obtaining bright and saturated color images. In particular, U.S. Pat. No. 4,364,640 describes a device for capturing, guiding and concentrating light extracted via an outlet window comprising at least one flexible foil made of a synthetic material containing a fluorescent component capable of converting ambient light into fluorescent light. The device can include second and the third flexible foils transmitting the light outgoing from the outlet window. In one embodiment, a pair of foils is arranged behind a liquid crystal display. Each foil contains a fluorescent material and their polarization planes are perpendicular to each other. An optically active layer placed between the two foils rotates the direction of polarization. The light is extracted through a foil on the side opposite to the liquid crystal cell, so that a viewer sees the light modulated by the cell.
Japanese Patent 60:061,725 describes a color LCD with increased brightness. The device employs a fluorescent material, in which emission is excited by visible light of a short wavelength, instead of using a color filter.
Devices of another type employ polarized light for illuminating LCDs. In such devices, the light polarizers are arranged on the surface of elements of the backlight system or between the backlight system and a liquid crystal cell. For example, a surface light source device with polarization function comprises a fluorescent lamp enclosed by a silver foil sheet from which a parallel illuminating light flux is extracted through a light exit surface and a polarization converter. The polarization converter enhances the polarization function of the surface light source device through polarization conversion action accompanying reflection in a prism. An intensely bright polarized illuminating light flux is extracted through a light exit surface. When such a backlight system is applied in an LCD, an exit light direction modifier is arranged outside the LCD panel.
Another means of illuminating LCDs with a polarized light is offered by a backlight unit comprising a lamp and a set of prisms arranged in between the light guide plate and reflecting film, projecting and condensing light from one side of the light guide reflector. The prisms have a prism angle such that the angle between the direction of light incidence onto the surface of the prism and the normal to the surface is equal to the Brewster angle. The prisms are arranged so that they are parallel to the direction of the polarization axis of the polarizing plate on the bottom surface of the liquid crystal panel.
The most effective system capable of converting all the nonpolarized incident light flux into polarized light with minimum losses is offered by the so-called optical recycling scheme with reflective polarizer.
The reflective polarizer usually comprises a multilayer structure consisting of alternating anisotropic and isotropic layers, with the refractive index of an isotropic layer being equal to that of one of the anisotropic layers. This structure is capable of transmitting light in one polarization state while reflecting light of the perpendicular polarization. In one of such structures, the reflected polarized light passes through a quarter-wave plate, changes the polarization direction, reflects from a mirror, then again passes through a quarter-wave plate, and enters the reflective polarizer again, this time in the first polarization state.
Another variant of the optical recycling scheme stipulates depolarization of the light component reflected from the reflective polarizer. This can be achieved, for example, by using a diffuse reflector. Upon reflection, the nonpolarized light enters the reflective polarizer.
The aforementioned methods and LCD design solutions are aimed at improving separate characteristics of backlight systems, rather than at solving the general problem of obtaining a bright high-efficiency source of polarized radiation possessing required spectral properties and desired dimensions (including those intended for use in thin-film devices).